Troubleshooting at Crate gfx 212

[telebob]

Hi, new to the forum so hello to all.

I'm working on a crate gfx 212 that when plugged in with a guitar has no output. It does not light my current limiter when I turn it on. I have run test signal through and have signal through all of the pots. After turning off my test tone tracer I noticed that the speakers on the amp are producing the tone at a super low volume. I have signal although a low volume at the output going to the speakers as well. I'm also getting a low level signal trace when I touch the heat. I'm not sure how I am getting signal off the grounded side if I can't find or don't show a direct short. I have found one diode that does not test correct but it is in the footswitch section of the schematic pretty far removed from the power section. I will replace it but should I just go ahead and replace all the power transisitors? Also they are tip142 and tip147 s respectively where is a good source to find good parts with fair shipping? As you can tell I'm a bit new to this.

Thanks,
Tracy

[Enzo]

If your car wouldn't run, would you buy new tires? Unless you have a reason to replace those transistors, it is just guessing. And highly unlikely to be the problem.

Should be an INSERT jack on the panel, it is your FX loop. Plug the guitar or signal source into that. Does it come out the speakers? If so, the power amp is OK.

Also, you can plug a cord into the first notch of that jack and use it as an output to some other amp. Apply a test signal to the regular input, then run this insert out to some other amp for a listen. Does that sound OK out the other amp? That tests your preamp.

[km6xz]

All of what Enzo wrote...
Is the weak signal distorted or does it sound normal but just very quiet? When you turn it on or off does the speaker give a pop or and hiss if you put your ear to the speaker?

[Jazz P Bass]

To add, there have not been any test voltages posted.

Crate has excellent schematics in that respect.

Testing for expected power supply voltages is a good start.

After that the signal voltages (Vac) can be measured.

GFX-212A.zip

[The Dude]

Firstly, welcome to the place. Secondly, start with what Enzo suggested and check at the insert jack for a proper send and return. Another way to test this is to insert a stereo plug and jumper tip and ring together to see if the amp works. If it does, the problem is in your switching jack(s).

[telebob]

thanks for hopping in guys,

I ran test signal into the insert jack and got no sound in the speakers, I ran test signal into the input and using an insert cable got a weak signal on the tip of one lead and a louder but distorted signal out of the other. When I plugged each of the insert output leads into another amp I got the same thing, one side clean and not real loud and the other side distorted and stronger.

[Enzo]

The one side of your insert is the preamp out, the other side is the return to power amp. The weak signal there is just bleed through from the other side.

So your PA seems to have the issue.

This is a combo, right? Do you have the EXTERNAL SPEAKER jack on it? Try plugging a speaker into that jack. There are cutout contacts on that jack that complete the speaker circuit, and if they get dirty, no sound out the speaker.

Look at the PA input, see R48 and below it Q9, the JFET? That transistor is a mute. It needs to have voltage on its gate to turn it OFF. Is there any? Or apply a test signal and see if it is on the left end of R48, and then on the right end. If the signal disappears after R48, check that JFET, with the amp ON, measure resistance across R44, which is parallel to it. If you measure a low resistance, the JFET is probably killing you. if you measuer something like the 27k, it is likely OK.

The schematic has a number of test points, start with #9 and see how the later ones stack up.